The present invention relates generally to automotive systems and more particularly to seat sensor calibration methods and systems.
Recently, automotive manufacturers have developed Passenger Occupant Detection Systems (PODS) to aid in the decision making for deploying passenger side air bags.
Typically, these systems consist of a flexible laminate mat with printed resistors and cylindrical force concentrators. The force concentrators (FCDs) are bonded to the sensor mat over the printed resistors. The sensor mat is placed in the passenger seat between the seat foam and seat trim cover. When the surface of the seat is depressed, the sensor mat and printed resistors will wrap around the force concentrators, which creates a change in the circuit resistance. Based upon the pattern and the weight of the depression, the PODS system can determine whether or not to deploy the passenger side air bag in a crash situation.
PODS sensors must be calibrated in order to obtain optimal performance. There are several variables that can affect sensor performance, including material considerations associated with the seat and sensor components, product material or process variations, and crack preconditioning process. As a result of the multiple sources of variation, the PODS sensors must be calibrated after the seat bottom has been assembled.
The goal of the calibration is to quantify performance of the PODS when a load is applied to the seat surface. The basic calibration steps are to measure a sensor""s value in an unloaded state, apply a known load to the seat surface, and record the sensor value in the loaded state. The sensor performance data and the seat model information is then used to calculate the sensitivity of the sensors. The challenge has been to develop a method for applying the load to a seat that produces more reliable, repeatable, and realistic calibration data.
There have been several methods and pieces of equipment used in an attempt to acquire good PODS calibrations. One method focuses on applying a known force to each seat sensor. The seat is contacted with a flat 1.25xe2x80x3 diameter swivel pad usually made of steel or delrin material. The system uses a robotic motorized Z-axis to apply force to the seat. A force sensor integrated into the Z-axis monitors the applied force. The Z-axis drives into the seat until the desired force level is achieved. While the system provides acceptable laboratory results, additional testing uncovered problems controlling the force that was applied to the seat surface. Noise tended to generate erroneous force readings. The serving nature of the robot""s Z-axis also introduced instabilities to force sensor values. Finally, the system exhibited sensor registration problems associated with off-center contact on the seat sensor and non-uniform and non-planar seat foam compression.
New concepts were developed in response to the problems associated with the original calibrators. An air spring assembly was implemented to design the force sensor out of the system. The air spring produces constant force over its entire length of travel. Because of this fact, the robot is able to drive the Z-axis to a nominal depth and be certain of contacting the seat with known force regardless of the height variation of the seat surface.
In response to the registration issues, the steel/delrin swivel pad was replaced with a 6xe2x80x3 urethane-nylon laminate air bladder. Testing concluded that contacting the seat with a flaccid air bladder would produce acceptable sensitivity to seat sensor registration, allowing mis-registration up to +/xe2x88x9225 mm without adversely affecting the sensor values. Further, the air bladder is able to conform to non-planar surfaces and non-perpendicular surfaces, such as sensor locations in seat bolsters.
However, while the air bladder created certain improvements, it also created additional problems. Assuming the contact force is always the same, the pressure on the seat surface would be fairly repeatable from seat sensor to seat sensor and machine to machine. With the air bladder design, however, the contact area was a function of the amount of air trapped in the bladder, the size of the bladder, air temperature, and the seat surface contour.
Methods were created to attempt to define and control these parameters. One method was to fill the bladder to a predefined pressure while pressing the bladder against a flat and rigid surface under a known force. Another method was to fill the bladder to a pre-determined height while pressing the bladder against a flat and rigid surface under a known force. These set-up methods attempted to control the amount of air in the air bladder, but did not solve production problems on non-planar seat surfaces and did not directly control the air bladder pressure.
Other calibration methods were attempted, and all of the methods possessed several common weaknesses. First, because all of the methods were single point calibrators, only one seat sensor could be calibrated at one time. As a result, it could take 2 to 4 minutes to calibrate a single-seat.
Second, testing indicated that a seat sensor""s final position could move upwards of +/xe2x88x9235 mm from its nominal position due to uncontrollable seat assembly process and the nature of the materials used to construct the seat. Off-center actuation of the seat sensors produced unrepeatable and unreliable calibration date.
Third, since air was trapped in the 6xe2x80x3 diameter bladder, ambient temperature changes resulted in pressure changes within the air bladder, resulting in inaccurate sensor readings.
Fourth, integration of the robotic calibrators into different manufacturing lines for use on a wide variety of seat models, seat manufactures, and seat assembly processes would require custom calibration solutions in almost every instance.
Fifth, setting up, controlling, and monitoring process parameters was extremely difficult and time consuming. The interdependentness of air spring force and air bladder volume process parameters would require adjustments in an iterative manner to bring them within limits. In addition, it would be difficult to create a xe2x80x9ctransferxe2x80x9d standard that could be used to validate the operation of all machines.
As a result, a need exists for an improved method and system for calibrating seat sensors that produces reliable and repeatable results in a simple and efficient manner.
While the above methods for calibration focused on applying a controlled force to each seat sensor position with a different apparatus to contact the seat, the present invention focuses on two premises. First, it will be assumed that seat foam acts as a compressible fluid. Second, the amount of sensor mat wrap around an FCD and glue joint is directly related to seat surface pressure. A higher seat surface pressure would cause more sensor mat wrap thus generating a higher seat sensor value. In short, a consistent surface pressure acting on the seat surface will produce consistent reaction pressure in the seat foam, consistent seat deformation, and consistent and reliable sensor data.
The present invention incorporates these two premises into a calibration device generally referred to herein as a Big Bag Calibrator. On the Big Bag Calibrator, air bladder pressure is monitored and controlled directly. A precision pressure sensor is pneumatically tied to the air bladder and relays pressure information back to a computer. The computer determines whether to add or exhaust air to maintain the target pressure within the bladder. With the Big Bag Calibrator, a controllable pressure is applied to the entire seat surface that actuates all of the seat sensors in a consistent and repeatable manner.
This process eliminates the need for all of the indirect monitoring steps required for the prior art calibrators. Further, the process offers several advantages over the prior art. Since the Big Bag Calibrator tests all seat sensors at one time, the cycle time for testing seat sensors is substantially reduced. The Big Bag Calibrator also is simpler and less expensive to maintain than previous calibrators are. The expensive steel base, robot, servo amplifiers, motion control cards, and related control software are replaced with an inexpensive base and air cylinder. Further savings are realized in associated maintenance, downtime, and training. Further, the Big Bag Calibrator is relatively insensitive to sensor registration because the air bladder extends well beyond the edges of a typical car seat.
Other advantages include the fact that ambient temperature changes would not affect system performance, since the air bladder pressure is controlled directly. Also, the Big Bag Calibrator is easy to integrate into a wide variety of seat models, seat manufacturers, and seat assembly processes. Seats can be calibrated as individual seat bottoms, as full seats, or on a pallet containing a car""s full complement of seats. These may be run off a single, machine without significant hardware or software changes. Finally, the Big Bag Calibrator directly and accurately controls the pressure applied to the seat surface, eliminating many unnecessary control problems.